Shallow trench isolation (STI) structures are often used in semiconductor devices for providing electrical isolation among transistors. The fabrication process of STI structure may produce cosmetic defects such as trench cones, which typically do not impact the electrical isolation capability of the STI structures. As the demand for high voltage devices surges in recent years, STI structures have been integrated to form high voltage devices, in which the STI structures may be adapted as a high voltage barrier between an upper conductive plate and a lower substrate. In certain configurations, trench cones may alter the average thickness of an STI structure, which may affect the high voltage performance of the integrated device.